The overall objectives are to develop criteria for characterization of metal sites in various metalloproteins using variable-temperature magnetic susceptibility, electron paramagnetic resonance, Fe57 Mossbauer, infrared, electronic absorption, and X-ray photoelectron spectroscopies. In addition to"model" systems, metalloproteins of direct or indirect interest include mushroom tyrosinase, ferredoxins that are potentially mixed valent such as reduced putidaredoxin, spin equilibria systems such as various electron transport systems, particularly cytochrome-c. In an effort to understand the factors determining whether a given system is mixed valence or not, the thermal "intervalence" electron transfer rates are being gauged for a series of singly-oxidized metallocenes of iron and cobalt. It is proposed that the level of exchange-type interaction is important, and the exchange parameter in H equals minus 2J S sub 1 times S sub 2 is being determined for higher or lower oxidation derivatives of the same metallocenes. Mixed-valence work is being carried out in the singly-oxidized compounds derived from various oxobridged Fe(III) dimers. Several solid state "models" for electron transfer at the heme site of cytochrome-c are being investigated. The vibronic nature of certain 6A sub 1/2T sub 2 spin equilibria in Fe(III) complexes is being investigated. Lyophilized samples of mushroom tyrosines are being studied with variable-temperature susceptibility. BIBLIOGRAPHIC REFERENCES: W. H. Morrison, Jr., E. Y. Ho, and D. N. Hendrickson, "Reaction of Ferrocene with Polyaromatic Molecules. Pi-Arene-bis-(pi-cyclopentadienyliron) Dications. II. Electronic Structure," Inorg. Chem., 14, 500 (1975). C. G. Pierpont, D. N. Hendrickson, D. M. Duggan, F. Wagner, and E. K. Barefield, "Crystal and Molecular Structure of Di-micron-azido-bis-(2,2',2"-triaminotriethylamine) dinickel(II) Tetraphenylborate. Magnetic Exchange Between Azide-bridged Octahedral Nickel(II) Centers, the Di-micron-azido and Mono-micron-azido Cases," Inorg. Chem., 14, 604 (1975).